On a Remarkable Cube of Pyrite, Carrying Crys- Tallized Gold and Galena of Unusual Habit

ON A REMARKABLE CUBE OF PYRITE, CARRYING CRYS- TALLIZED GOLD AND GALENA OF UNUSUAL HABIT By JOSEPH E. POGUE Assistant Curator, Division of Mineralogy, U. S. National Museum With One Plate The intergrowth or interpenetration of two or more minerals, especially if these be well crystallized, often shows a certain mutual crystallographic control in the arrangement of the individuals, suggestive of interacting molecular forces. Occasionally a crystal upon nearly completing its growth exerts what may be termed "surface affinit}'," in that it seems to attract molecules of composition differ- ent from its own and causes these to crystallize in positions bearing definite crystallographic relations to the host crystal, as evidenced, for example, by the regular arrangement of marcasite on calcite, chalcopyrite on galena, quartz on fluorite, and so on. Of special interest, not only because exhibiting the features mentioned above, but also on account of the unusual development of the individuals and the great beauty of the specimen, is a large cube of pyrite, studded with crystals of native gold and partly covered by plates of galena, acquired some years ago by the U. S. National Museum. This cube measures about 2 inches (51 mm.) along its edge, and is prominently striated, as is often the case with pyrite. It contains something more than 130 crystals of gold attached to its surface, has about one-fourth of its area covered with galena, and upon one face shows an imperfect crystal of chalcopyrite. The specimen came into the possession of the National Museum in 1906 and was ob- tained from the Snettisham District, near Juneau, Southeast Alaska. It is now on exhibition in the Mineral Department under number 86045. Three similar specimens were exhibited at the Seattle Expo- sition during 1909, one of which is stated by the owner, Mr. L. V. Winter, of Juneau, Alaska, to be 4 inches square and to show 170 crystals of gold upon its surface. So far as can be learned the four specimens are the sole representatives of a very unique association. Crystallography of the pyrite.—The pyrite has four of its faces well developed ; each of the two remaining ones is marred by an irregular pit, about one-half inch deep and the same in diameter, and the edge joining these two faces is imperfect. The crystal is striated parallel to the pyritohedron e (210), due to oscillatory com- 477 ; 478 SMITHSOXIAX MISCELLANEOUS COLLECTIONS VOL. 52- bination of this form and the cube a (lOO), which gives rise to overlapping strips or laminse parallel to (lOo) and bounded by (210), each lower strip being usually a bit broader than the one above. The ordinary arrangement across the laminations is: A smooth sur- parallel to face, i face the cube to 2 mm. in width ; a series of steps down across alternations of cube and pyritohedron for about i mm. Fig. 81. — Pyrite cube with crystallized gold and galena. Natural size. a flat-bottom valley i to 2 mm. wide; a course of steps up for i mm. to a second plane surface, and so on. Frequently a narrow strip extends only partly across the cube face when, cut off by two octahedral planes, it ends in a dull point, and the strip beneath continues until it perhaps is terminated by similar bounding planes. Scharff^ ' Scharff, F. Ueber die Bau-weise der Wiirfel-formigen Krystalle. Neues Jahr. f. Min., etc., 1S60, pp. 385-425. See especially p. 412 and Figs. 41, 43, 47, Plate VI. : A'U. 1882 PVRITli CAKKVIM. r,:)Ll) AXU GALENA POGUli 479 has figured and described natural etchings on p3'rite from Traver- sella, Italy, which are somewhat similar to the ones here depicted. At times the gold crystals or small rounded knobs of galena are situated upon small six-sided pedestals composed of laminae of pyrite bounded by two pyritohedron and four octahedral planes. The above features may be seen by referring to plate Lix, figure i. CrystallograpJiy of the gold.—The gold crystals are most abundant on the face shown in plate ijx, figure i, though some are present on each of the other faces. They are usually from one-third to one- half buried in the pyrite. nevel- more, and seem to have no definite orientation in regard to their host. Most of them show crystal outline and many are rather synmietrically developed ; their average diameter is about i mm. The faces are slightly convex, without bright luster, and the edges are not sharp. No measurements were attemptefl on the goniometer, as the crystals were not fitted for giving reflection, nor, indeed, could they be easily plucked from their settings. The following forms, however, by aid of a hand lens, were Fig. 82. —The most common shapes of the gold crystals. Enlarged octahe- ; combination of and dron ; combination of cube and octahedron octahedron •dodecahedron ; combination of octahedron and trapezohedron. positivelv determined: cube, a (lOo) ; octahedron, o (m) : dodecahedron, d (no), and trapezohedron, n (211) or m (311). To these should be added a hexoctahedron as probably present, this form pos- sibly corresponding to .r (18.10.1) described by Dana^ on gold from California. The most common combinations, as shown in figure 82, are : Octahedron, cube and octahedron, dodecahedron and octahedron, and trapezohedron and octahedron. Crystallography of the galena.—The galena possesses three distinct habits, two of which have two or more appearances, due to peculiarities of orientation (i) Normal galena. A very small part of the galena has the ordinary step-like appearance characteristic of this mineral and re- quires no special description. This phase is arranged with one cubic cleavage parallel to the cube faces of the pyrite, with the striations on the latter intersecting diagonallv its other cleavages. ° Dana, E. S. On the crystallization of gold. Am. Jour. Sci., vol. 32, 1886, -pp. 132-138. 48o SAIITHSOXIAN MISCELLANEOUS COLLECTIONS VOL. 52- (2) Galena laminated parallel to the octahedron (m). About •one-fourth of the mineral is developed in this way and is arranged with its octahedral surface usually parallel, though at times slightly inclined, to the cube faces of the pyrite. Natural etching has given a triangular and hexagonal outline to the plates, as is shown in the lower right-hand corner of figure 2, plate lix. This contour is explained by the fact that an octahedral plane alone is an equilateral triangle, and, when truncated by cube faces, forms a surface of six- sided outline. The strongly laminated nature of the galena may be due to polysynthetic twinning parallel to (m). (3) Galena laminated parallel to the cube (100). This habit, which comprises about two-thirds of the galena, shows a varied orientation in respect to the pyrite. (a) The most common appearance is shown in the central portion of figure 2, plate lix, where the laminae are parallel to the surface of the pyrite. This is explained Fig. 83. —Greatly enlarged cleavage fragment of galena, showing eminent Fig. 84. —Fragment of galena, made cubic cleavage modified bj' octahedral up of laminae, parallel to a cube face, cleavage. bounded by octahedral slopes. by the accompanying drawing (figure 84), which figures a fragment made up of plates parallel to the cube a (100), bounded by octahedral (hi) slopes, this combination giving a square outline to the plates, (b) Occasionally there occur long, narrow strips, likewise made up of laminae parallel to a (100) and bounded by elongated (ill) faces. These may have their a (100) faces parallel to a (too) of pyrite; or less often the long o (11 1) faces may have this relation. Crystals of like distortion, but without such platy structure or appearance, from Yellow,stone, Wisconsin, have been described by Hobbs.^ (c) Finally, the laminae and the two cube * Hobbs, W. H. Die krystallisirten Mineralicn aus dem "Galena Limestone" des siidlichen Wisconsin und des nordlicben Illinois. Zcitsch. fiir Kryst.. vol. 25, i895-'96, pp. 257-275. Especially plate 4, figure 10, and p. 263. NO. 1882 pykite; carrying gold and galena—pocue 481 faces at right angles to these may all be equally inclined to the surface of the pyrite. The mineral with this arrangement has a rhom- bohedral appearance (imperfectly shown in the upper central portion of figure 2, plate Lix), but its true nature is revealed by expos- ing the cleavages, which are parallel to the external planes. At times, as is shown in plate Lix_, figure i, irregular branching forms, suggestive of fantastic figures, result from this orientation. The preceding conclusions were arrived at by a study of dozens of cleavage fragments under the microscope and an examination with hand lens of the galena in place, the prominent cubic cleavage in all cases serving as a means of orientation. Measurements of the cleavage by the microscope gave 903^°, 89^/2°, 90°, 89°. Several fragments showed secondary octahedral cleavage, which is rare for galena. One example is pictured in figure 83, in which a corner of Fig. 85. —Greatly enlarged cleavage surfaces of galena, showing groovmgs visible by incident sunlight under the microscope. a cleavage cube is broken across by a series of smaller cubes, with their corners, in turn, truncated by minute triangular octahedral faces. An examination of cleavage surfaces under a high magnifying power, illuminated by incident sunlight, reveals a complicated series of striations and groovings. The striations are exceedingly minute, visible as fine hair-lines only under the most favorable conditions of reflection. There are two sets at right angles to each other and parallel to the edges of the square cleavage fragments, which may represent incipient cleavages ; a third set, less distinct, is sometimes present, cutting the cubic cleavage and striations diagonally.

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